Ford Escape: Exhaust System / Description and Operation - Exhaust System - System Operation and Component Description

System Operation

Catalyst And Exhaust Systems

The catalytic converter and exhaust systems work together to control the release of harmful engine exhaust emissions into the atmosphere. The engine exhaust gas consists mainly of nitrogen (N), CO2 and water (H2O). However, it also contains CO , NOX , hydrogen (H), and various unburned HC . The major air pollutants of CO , NOX , and HCs, and their emission into the atmosphere must be controlled.

The exhaust system generally consists of an exhaust manifold, a front exhaust pipe, a universal HO2S , a rear exhaust pipe, a catalyst HO2S , a muffler, and an exhaust tailpipe. The catalytic converter is typically installed between the front and rear exhaust pipes. On some vehicle applications, more than one catalyst is used between the front and rear exhaust pipes. Catalytic converter efficiency is monitored by the OBD system strategy in the PCM . For additional information on the OBD catalyst monitor, refer to the description for the OBD catalyst monitor in this section.

Only 2 heated oxygen sensors are used in an exhaust stream. The universal HO2S is before the catalyst (universal HO2S11 or universal HO2S21) and used for primary fuel control while the rear HO2S is after the catalyst (HO2S12 or HO2S22) and used to monitor catalyst efficiency.

Catalytic Converter

A catalyst is a material that remains unchanged when it initiates and increases the speed of a chemical reaction. A catalyst also enables a chemical reaction to occur at a lower temperature. The catalytic converter assists in controlling the concentration of exhaust gas products released to the atmosphere. It contains a catalyst in the form of a specially treated ceramic honeycomb structure saturated with catalytically active precious metals. As the exhaust gases come in contact with the catalyst, they are changed into mostly harmless products. The catalyst initiates and speeds up heat producing chemical reactions of the exhaust gas components so they are used up as much as possible.

Light Off Catalyst

As the catalyst heats up, converter efficiency rises rapidly. The point at which conversion efficiency exceeds 50% is called catalyst light off. For most catalysts this point occurs between 246°C to 302°C (475°F to 575°F). A light off catalyst is a TWC converter that is located as close to the exhaust manifold as possible. Because the light off catalyst is located close to the exhaust manifold it achieves the required temperature faster and reduces emissions more quickly than the catalyst located under the body. Once the catalyst lights off, it quickly reaches the maximum conversion efficiency for that catalyst.

Three Way Catalytic (TWC) Converter Conversion Efficiency

A TWC convertor requires a stoichiometric air fuel ratio of 14.7 pounds of air to 1 pound of gasoline, or 14.7 to 1, for high conversion efficiency. To achieve these high efficiencies, the air to fuel ratio must be tightly controlled with a narrow window of stoichiometry. Deviations outside of this window greatly decrease the conversion efficiency. For example a rich mixture decreases the HC and CO conversion efficiency while a lean mixture decreases the NOX conversion efficiency.

For vehicles using E85 the required air to fuel ratio is 9.8 to 1. For vehicles using E100 the required air to fuel ratio is 9 to 1. Other gasoline/ethanol mixtures require a variable air to fuel ratio between 14.7 to 1 to 9.8 to 1 dependent on the percentage of ethanol content.

Exhaust System

The exhaust system conveys engine emissions from the exhaust manifold to the atmosphere. Engine exhaust emissions are directed from the engine exhaust manifold to the catalytic converter through the front exhaust pipe. A universal HO2S is mounted on the front exhaust pipe before the catalyst. The catalytic converter reduces the concentration of CO , unburned HCs, and NOX in the exhaust emissions to an acceptable level. The reduced exhaust emissions are directed from the catalytic converter past another HO2S mounted in the rear exhaust pipe and then on into the muffler. Finally, the exhaust emissions are directed to the atmosphere through an exhaust tailpipe.

Underbody Catalyst

The underbody catalyst is located after the light off catalyst. The underbody catalyst may be in line with the light off catalyst, or the underbody catalyst may be common to 2 light off catalysts, forming a Y pipe configuration.

Three Way Catalytic (TWC) Converter

The TWC converter contains either platinum (Pt) and rhodium (Rh) or palladium (Pd) and rhodium (Rh). The TWC converter catalyzes the oxidation reactions of unburned HCs and CO and the reduction reaction of NOX . The 3 way conversion can be best accomplished by always operating the engine air fuel ratio at or close to stoichiometry.

Exhaust Manifold Runners

The exhaust manifold runners collect exhaust gases from engine cylinders. The number of exhaust manifolds and exhaust manifold runners depends on the engine configuration and number of cylinders.

Exhaust Pipes

Exhaust pipes are usually treated during manufacturing with an anti corrosive coating agent to increase the life of the product. The pipes serve as guides for the flow of exhaust gases from the engine exhaust manifold through the catalytic converter and the muffler.

Heated Oxygen Sensor (HO2S)

The HO2S provides the PCM with information related to the oxygen content of the exhaust gas.

Muffler

Mufflers are usually treated during manufacturing with an anti corrosive coating agent to increase the life of the product. The muffler reduces the level of noise produced by the engine, and also reduces the noise produced by exhaust gases as they travel from the catalytic converter to the atmosphere.

Catalyst Efficiency Monitor

The catalyst efficiency monitor uses an oxygen sensor before and after the catalyst to infer the HC efficiency based on the oxygen storage capacity of the catalyst. Under normal closed loop fuel conditions, high efficiency catalysts have significant oxygen storage. This makes the switching frequency of the rear HO2S very slow and reduces the amplitude, which provides for a shorter signal length. As the catalyst efficiency deteriorates due to thermal and chemical deterioration, the catalyst ability to store oxygen declines. The post catalyst or downstream HO2S signal begins to switch more rapidly with increasing amplitude and signal length. The predominant failure mode for high mileage catalysts is chemical deterioration (phosphorus deposits on the front brick of the catalyst) and thermal deterioration.

The catalyst monitor calculates the rear HO2S signal lengths for 10 to 20 seconds during part throttle, closed loop fuel conditions after the engine is warmed up, the inferred catalyst temperature is within limits, and fuel tank vapor purge is disabled. The catalyst monitor is enabled for 10 to 20 seconds per drive cycle. When the catalyst monitor is active, the PCM commands a fixed fuel control routine. During monitor operation the rear HO2S signal lengths are continually calculated. The calculated rear HO2S signal length is then divided by a calibrated signal length, which has compensation for mass airflow. The calibrated signal length is based on the signal length of an HO2S placed after a catalyst without a washcoat. An index ratio near 0.0 indicates high oxygen storage capacity and high HC efficiency. An index ratio near 1.0 indicates low oxygen storage capacity and low HC efficiency. If the actual index ratio exceeds the threshold index ratio, the catalyst is considered failed.

Inputs from the engine CHT or ECT , IAT , MAF , CKP , TP , and vehicle speed are required to enable the catalyst efficiency monitor.

Typical Monitor Entry Conditions:

• Minimum 330 seconds since start up at 21°C (70°F)
• Engine coolant temperature is between 76.6°C - 110°C (170°F - 230°F)
• Intake air temperature is between -7°C - 82°C (20°F - 180°F)
• Time since entering closed loop is 30 seconds
• Inferred rear HO2S temperature of 482°C (900°F)
• Part throttle, maximum rate of change is 0.2 volts/0.050 sec
• Vehicle speed is between 8 and 112 km/h (5 and 70 mph)
• Fuel level is greater than 15%
• First Airflow Cell
  • Engine rpm 1,000 to 1,300 rpm
  • Engine load 15 to 35%
  • Inferred catalyst temperature 454°C - 649°C (850°F - 1,200°F)
  • Number of universal HO2S switches is 50
• Second Airflow Cell
  • Engine rpm 1,200 to 1,500 rpm
  • Engine load 20 to 35%
  • Inferred catalyst temperature 482°C - 677°C (900°F - 1,250°F )
  • Number of universal HO2S switches is 70
• Third Airflow Cell
  • Engine rpm 1,300 to 1,600 rpm
  • Engine load 20 to 40%
  • Inferred catalyst temperature 510°C - 704°C (950°F - 1,300°F)
  • Number of universal HO2S switches is 30

Six drive cycles may be required to illuminate the MIL during normal customer driving, because an exponentially weighted moving average algorithm is used to determine a concern. If the KAM is reset, a concern illuminates the MIL in 2 drive cycles.

General Catalyst Monitor Operation

The catalyst monitor duration is 12 to 30 seconds, once per drive cycle. If the catalyst monitor conditions are met, the catalyst monitor may run and complete after all of the upstream HO2S functional tests are complete and the EVAP system is functional, with no stored DTCs; however, the catalyst monitor may run and complete before the downstream HO2S deceleration fuel shut off test is complete. In this case, the catalyst monitor inspection maintenance (I/M) readiness flag may indicate complete before the O2S I/M readiness flag indicates complete. If the catalyst monitor does not complete during a particular driving cycle, the already accumulated switch/signal data is retained in the KAM and is used during the next driving cycle to allow the catalyst monitor a better opportunity to complete.

Some vehicles that are part of the low emission vehicle (LEV) catalyst monitor phase in, monitor less than 100% of the catalyst volume. Often this is the first catalyst brick of the catalyst system. Partial volume monitoring is done on LEV and ultra low emission vehicle (ULEV) vehicles in order to meet the 1.75 emission standard. The rationale for this strategy is the catalyst nearest the engine deteriorates first, allowing the catalyst monitor to be more sensitive and illuminate the MIL correctly at lower emission standards.

Some applications use partial volume monitoring, where the rear HO2S is located after the first light off catalyst can or after the second catalyst can in a three can per bank system (a few applications placed the HO2S in the middle of the catalyst can, between the first and second bricks).

Index ratios for ethanol (flex fuel) vehicles vary based on the changing concentration of alcohol in the fuel. The threshold to determine a concern typically increases as the percent of alcohol increases. For example, a threshold of 0.5 may be used at E10 (10% ethanol) and 0.9 may be used at E85 (85% ethanol). The thresholds are adjusted based on the percentage of alcohol in the fuel. Standard fuel may contain up to 10% ethanol.

The PCM calibration prevents the catalyst monitor from running on a new vehicle until 60 minutes of time has accumulated with the catalyst temperature greater than 426°C (800°F) or 483 km (300 miles) have accumulated. A replacement PCM or updated calibration does not prevent the catalyst monitor from running.

• The HO2S can be located in various configurations to monitor different kinds of exhaust systems. Inline engines and V engines are monitored by their individual bank. A rear HO2S is used along with the front, fuel control universal HO2S for each bank. Two sensors are used on an inline engine and 4 sensors are used on a V engine. Some V engines have exhaust banks that combine into a single underbody catalyst. These systems are referred to as Y pipe systems. They use only one rear HO2S along with the 2 front, fuel control universal HO2S . The Y pipe system uses 3 sensors in all. For Y piped systems, the 2 front universal HO2S signals are combined by the PCM software to infer what the exhaust oxygen content would have been in front of the monitored catalyst. The inferred front exhaust oxygen content and the rear HO2S signal is then used to calculate the index ratio.
• The MIL is activated after the first concern is detected. When a concern is detected after a KAM reset, the MIL is activated after 2 concecutive key cycles.

Integrated Air Fuel Catalyst Monitor

The integrated air fuel catalyst monitor is an on board strategy designed to monitor the oxygen storage capacity of the catalyst after a deceleration fuel shut off (DFSO) event. The monitor determines the amount of fuel needed to drive the catalyst to a rich condition when starting from an oxygen saturated, lean condition. The monitor is a measure of how much fuel is required to force the catalyst from a lean to a rich condition. The monitor runs during catalyst reactivation following a DFSO event. The monitor completes after approximately 3 DFSO monitoring events have occurred.

Component Description